Cannot undo > 'Remove occurrence' in 'Repeat'-dialog

I've set a recurring event every 2 weeks (...14.August...28.August...11.September...)
The calendar is on [mailbox.org](https://mailbox.org/) and Maya connects via caldav.

Now I deleted one occurrence with Maya (28.August)...

...after that there is an exception in the repeat-setting-dialog of this event, but i cannot delete this.
It always reappears after i reopen Maya.

```
jar@HP-13:~$ inxi -S
System: Host: HP-13 Kernel: 4.15.0-32-generic x86_64 bits: 64 Desktop: Gnome
Distro: elementary OS 5.0 Juno
jar@HP-13:~$ io.elementary.calendar
[INFO 19:29:45.372872] Application.vala:154: Kalender version: (null)
[INFO 19:29:45.372901] Application.vala:156: Kernel version: 4.15.0-32-generic

(io.elementary.calendar:26515): Gtk-WARNING **: 19:29:45.388: Theme parsing error: sidebar.css:30:21: The :focused pseudo-class is deprecated. Use :focus instead.
Gtk-Message: 19:29:53.242: GtkDialog mapped without a transient parent. This is discouraged.
[WARNING 19:29:58.357884] CalendarModel.vala:162: Kalenderobjekt kann nicht verändert werden: Daten konnten nicht gesendet werden: HTTP-Fehlercode 412 (Precondition Failed)
```

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